This program consists of seven projects concerned with the function, development and modulation of mechanisms of synaptic transmission. One major aspect relates to the determination of transmitter choice (norepinephrine vs. acetylcholine) by newborn rat sympathetic neurons in cell culture and to the changes that occur in the molecular constitution of these neurons as they switch from adrenergic to cholinergic. This work will continue and also be extended to a study of adult sympathetic neurons in culture. A similar shift in trasmitter status appears to occur in vivo during development of the innervation of sweat glands; this change will be studied in the electronmicroscope to determine whether a single population of endings changes its neurotransmitter with time. Studies of the peptidergic input to frog sympathetic neurons will be continued in order to learn more about the ionic mechanisms activated by luteinizing hormone releasing hormone (LHRH) or by the LHRH-like natural transmitter. Studies of modulation of the properties of excitable cells in lobsters, by amine and peptide neurotransmitters and neurohormones, will be continued. Further studies will also be made of the development of sensory-motor connections in the spinal cord of the bull frog.